The present invention relates generally to anti-skid control systems for use in motor vehicles, and more particularly to such an anti-skid control system arranged so as to adjust the pressure of a wheel braking cylinder by a pressure control valve to prevent a locking of a wheel which can occur when the motor vehicle is running.
Conventionally, an anti-skid control system is known as disclosed in Japanese Patent Publication No. 51-6308 (U.S. Pat. No. 3,637,264), the technique of which is arranged such that the braking pressure to the wheel braking cylinder is adjusted in accordance with duty control of a two-position valve between the pressure-increasing position and the pressure-decreasing position.
Generally, such an anti-skid control system controls the braking pressure to the wheel braking cylinder in accordance with predetermined duty ratios for low pressure-increasing operation, low pressure-decreasing operation and pressure-maintaining operation. However, the pressure-increasing and pressure-decreasing slopes developed when the two-position valve takes the pressure-increasing position and the pressure-decreasing position depend upon the braking pressure in the master cylinder, the braking pressure in the wheel braking cylinder and others. At this time, in the duty control of the two-position valve, a duty ratio exsits whereby the pressure-increasing amount obtained in accordance with the pressure-increasing slope can become equal to the pressure-decreasing amount obtained in accordance with the pressure-decreasing slope. This balanced duty ratio causes maintaining the braking pressure in the wheel braking cylinder to be constant. Accordingly, in the case of fixing the duty ratios in advance as disclosed in the aforementioned prior art, even if the two-position valve is operated with the duty ratio set for the slow pressure-increasing purpose, for example, when the duty ratio is greater than the balanced duty ratio, the braking pressure in the wheel braking cylinder results in being decreased, thereby making it difficult to reliably and surely control the increase and decrease in the braking pressure in the wheel braking cylinder.